1. Field of the Invention
This invention relates to a corrosion-resistant material excellent in resistance to the corrosion and the thermal impact exerted by molten metal and suitable for use as in crucibles and a method for the production thereof.
2. Description of the Related Art
Rare earth metals such as lanthanum (La), yttrium (Y), neodymium (Nd), and terbium (Tb) are indispensable elements as new alloy materials for permanent magnets, photomagnetic materials, hydrogen absorbing alloys, etc. In recent years, in consequence of the growth of applications found for the alloy materials, the demands for these rare earth metals have been sharply increasing. These rare earth metals have in common the fact that they have lower levels of free energy for the formation of an oxide than the other metallic elements and their oxides are very stable chemically. They occur in the form of oxides in ores, therefore, and are extracted as pure metals from the ores through the processes of smelting and refining. The rare earth metals are used more often in the form of alloys with various metals than they are used as simple metals. During the processes of smelting and refining mentioned above and during the process of alloying as well, these rare earth metals are required to be held for a long time in their molten state in a crucible. Since the rare earth metals in their molten state are extremely active chemically and are liable to react with the material forming the crucible and melt and pass this material into the molten rare earth metals, they are at a disadvantage in inevitably increasing their contents of impurities.
In the materials used for the crucible, ceramic substances generally prove excellent in respect that they do not easily react with a molten rare earth metal. Since the ceramic substance is brittle and liable to be broken by thermal shock or thermal stress, however, the possibility that the molten rare earth metal will flow out of the crucible is high. Further, in the case of a crucible made of a ceramic substance, it often happens that small fragments of the crucible material shed from fine cracks inflicted by thermal stress on the crucible mingle into the molten rare earth metal and add to its content of impurities. As materials, the crucibles made of ceramic substances enjoy excellent corrosion resistance and nevertheless suffer from poor reliability. Though they fit small crucibles of the laboratory grade, they cannot be adapted for large crucibles of the grade of commercial manufacture.
The ceramic coating of the inner wall surface of a crucible made of a metal as by means of plasma spraying is an effective method from the standpoint of repressing the reaction of a molten rare earth metal with the material of the crucible. Not infrequently, the ceramic layer which is brittle by nature sustains a crack owing to the difference in thermal expansion coefficient between the metal forming the crucible and the ceramic forming the coating. Since the ceramic coating layer sustains cracks and exfoliations during one cycle of service of the crucible, the metallic crucible similarly to the ceramic crucible is at a disadvantage in suffering small fragments shed from the ceramic coating to increase the content of impurities in the molten rare earth metal. Thus, the ceramic coating method is not practicable from the viewpoint of cost because the coating layer requires repair each time the crucible is used for melting a rare earth metal.
The refractory metals represented by tungsten (W) and tantalum (Ta) exhibit small degrees of saturated solubility to molten rare earth metals and excel in corrosion resistance besides possessing high melting points. Since these metals are tough as compared with ceramic substances, the possibility that the metallic crucible will sustain breakage from thermal shock or thermal stress and induce leakage of the molten rare earth metal from the crucible is small.
Under the present conditions, therefore, the practice of melting a rare earth metal on a commercial scale by the use of a crucible made of tungsten (W) or tantalum (Ta) is prevalent.
Even in the crucible which is made of tungsten, the fusion of tungsten as the material of the crucible into the molten rare earth metal cannot be thoroughly repressed. Further, in terms of service life, the crucible of tungsten barely tolerates a few cycles of service. From the viewpoint of lowering the cost, the desirability of imparting a long service life to the tungsten crucible has been finding growing recognition.
The present inventors formerly made a study on refractory metallic materials as to their behavior of corrosion in a molten rare earth metal and found that the corrosion occurs in two types of reaction mechanism as illustrated with a model in FIG. 3.
In one reaction mechanism, the corrosion is caused in the boundary between a refractory metal 1 as the material for a crucible and a molten rear earth metal 2 owing to the melting and the diffusion of refractory metal atoms 3 into the molten rare earth metal 2 (corrosion mechanism 1).
In the other reaction mechanism, the corrosion is caused selectively in a grain boundary 4 of the refractory metal 1 by the molten rare earth metal 2, with the result that the crystal grain of the refractory metal 1 will inevitably fall down into the molten rare earth metal 2 (corrosion mechanism 2). This reaction is a phenomenon of the order of crystal grains of the refractory metal 1, namely the order of such a large unit as some tens to some hundreds of .mu.m.
The corrosion reaction due to the corrosion mechanism 1 is a reaction which is necessarily governed by the combination of the refractory metallic material 1 used as the material for the crucible with the rare earth metal material 2 destined to be melted, the melting temperature, and the time. The corrosion of the refractory metal 1 caused by this mechanism, therefore, cannot be abated unless the combination of the materials is altered.
The corrosion reaction due to the corrosion mechanism 2 can be appreciably abated by improving the corrosion resistance of the grain boundary 4 of the refractory metal 1. In fact, the magnitude of the corrosion due to the corrosion mechanism 2 is several times the magnitude of corrosion caused by the corrosion mechanism 1. It has been ascertained, as a result, that the improvement of the corrosion resistance of the grain boundary 4 of the refractory metal 2 has a fair possibility of notably decreasing the amount of the crucible material to be melted into the molten rare earth metal 2 and consequently attaining the elongation of service life of the crucible.
The present inventors have been also ascertained that the infiltration of the molten metal into the grain boundary of the refractory metal can be repressed and the corrosion resistance offered by the refractory metal to the molten metal can be improved by a method which comprises causing ceramic particles to be dispersed in the grain boundary of the refractory metal by means of powder metallurgy (Japanese Patent Laid-Open Application No. Hei-02(1990)-73,944).
After various studies continued thence, it has been found that depending on the combination of the ceramic particles to be dispersed and the rare earth metal to be melted, this method is not fully effective in bringing about the improvement aimed at.